Electrophotographic photoconductors for use in image-forming apparatuses using electrophotographic methods include inorganic photoconductors including photosensitive layers composed of inorganic materials, such as selenium; and organic photoconductors including photosensitive layers mainly composed of organic materials, such as binder resins, charge-generating materials, and charge transport materials. Among these photoconductors, organic photoconductors are widely used because they are easily produced, materials for photosensitive layers can be selected from a wide variety of materials, and high design flexibility is provided, as compared with inorganic photoconductors.
Examples of organic photoconductors include single-layer organic photoconductors each provided with a photosensitive layer that contains a charge-generating material and a charge transport material in the same layer. It is known that single-layer organic photoconductors have simple layer structures, are easily produced, and suppress the occurrence of coating defects, as compared with multilayer organic photoconductors each including a charge-generating layer containing a charge-generating material and a charge transport layer containing a charge transport material stacked on a conductive base. Because of these advantages, single-layer organic photoconductors are increasingly being used.
Furthermore, it is known that the use of contact charging methods in positive polarity serving as methods for charging electrophotographic photoconductors of image-forming apparatuses significantly reduces the amount of oxidizing gas generation, such as ozone, formed during charging. So, a positive charging method in positive polarity is often employed as a method for charging an electrophotographic photoconductor of an image-forming apparatus in view of adverse effects on the lifetime of photoconductors and office environments by the emission of the oxidizing gas, such as ozone. Positive charging single-layer electrophotographic photoconductors are increasingly being used from this point of view.
However, photosensitive layers of organic photoconductors are liable to wear because they are composed of soft organic materials. It is known that, in particular, surface states are easily changed due to wear during the initial use of organic photoconductors, thus causing a rapid reduction in surface potential with wear, so that image defects are liable to occur.
Additionally, in such contact charging methods, photosensitive layers wear significantly, compared with non-contact charging methods. In the case where positive charging single-layer electrophotographic photoconductors are used in contact charging methods, surface potentials are reduced during initial use, so that sufficient characteristics for use in an image-forming apparatus are not obtained.